IntroductionPearl is a natural gem and is produced by a mollusk whichhave a worldwide demand and have been exploited fromthe time immemorial.The entire trade was totally depending on the natural pearlresources causing heavy of damage to the naturalpopulations.To produce a pearl by the pearl oysters hit required at least3 to 4 years under natural conditions.In 1907 Tokichi Nishikawa produced the first sphericalcultured marine pearl in the oyster and the pearl cultureindustries started flourishing with a master touch ofMikimoto and several others.
The success in developing the technology of pearlproduction was achieved only in 1973 when the first batchof spherical cultured pearls was produced at CentralMarine Fisheries Research Institute.The world trade of cultured pearl is reported to be overUS$ 3 billion per year.The major pearl producing countries are Australia, Tahiti,Indonesia, Japan and China.India is one of the major importers of pearls importingpearls worth US $ 4 million every year to meet the growingdomestic market demand.
Method of pearl culturePearl culture operations can be divided into threecategories which are(1) collection/hatchery production,(2) on growing and(3) pearl culture.Each phrase of production permits a degree ofspecialization by farmer and allows people of differentincome and different technical expertise to get involved incultivation.Collection/ hatchery productionThe pearl oyster industry relies on spat collection atnatural production atolls where spat is abundant, duringthe warm season (November to May).Farmers place their collectors, made of black shade mesh
Spat oysters are left on collectors for 1 year till 4- 5cm inlength where they are big enough to be grafted, and skipthe on growing process altogether.Some of the spat are produced in hatcheries To induce spawning, oysters are held overnight at 20 0C,and then transferred to a raceway with the seawatertemperature at 30–32 0C. Spawning oysters are removed individually into individualcontainers. Larvae are reared in static water exchange systems wherewater is changed every 2-3 days. At day 20, when most of them reach 150μm, they aretransferred to seawater settlement tanks.
On growing Hatchery grown juveniles are put into the sea on the materialwhich they settled upon. The material is hung from long lines in areas of calm water. The spat are left to grow for 2 years till an average size of 90mm. A hole is then drilled through the posterior ear and the oystersare hung down in pairs on a down line.Pearl culturePearl culture involves theimplantation of a spherical nucleus together with a piece ofmantle tissue (Saibo) from a sacrificial oyster, into the gonads.The mantle tissue grows around the nucleus and secretesnacreous deposits to form a pearl.
At a minimum size of 100mm, the preoperative pearloyster undergoes a weakening process for 40 days for themuscular and gonadal epithelium to disintegrate. The oysters are then returned to the sea and placed incalm conditions. Cells in the pearl sac secretes nacre onto the outer surfaceof the nucleus and cultured pearls are harvested whennacre are 1 mm thick after 18-24 months.
Farming methodsRaft cultureMost suitable and appropriate one to farm the oysters inthe sheltered Bays.Wooden poles lashed with coir ropes and floated with thehelp of buoys moored by 2 to 4 anchors tied by means of15mm nylon ropes.A raft of 6 x 5 m is found to be more ideal for the pearloyster, culture and can accommodate 80-100 cages of40 x40 x 10 cm in size.
Collapsible or long line raft Comprised of 16 to 20 empty PVC barrel (200 litrecapacities) arranged in a row or 4 rows connecting oneanother by a chain stretched, and anchored to all the ends. Between, the gaps, the pearl oyster cages are suspended. Suspended cages get entangled during the rough weatherand get damaged due to hitting with one another.
Rack System 3" dia GI pipe materials or 5 ̋ bottom casurina or eucalyptuspoles erected at a depth of one metre into the soil in thesea with 2 to 3 m depth of water column at low tide. The poles are cross connected by means of wooden polesand tied by means of coir ropes. Accommodate 80 to 100 cages per compartments Can be extended to a long distance more than 200 m fromthe shore to a depth of3 to 5 m into the sea. Very convenient for the operation and also to attend thefarming work easily.
On bottom culture Possible only on the rocky sea bottom or artificial bottom In an area of 100 x 3 m, 300 oyster cages can be kept. Can be applied as a temporary means to accommodate thewild collected oysters before farming into off shore rafts.
Management methods Although pearl culture is extensive with little control overweather, the use of management methods can drasticallyincrease productivity and result in higher profitability. Therefore, management of culture system such as siteselection, settlement, feeding, stocking density and pearlculture technique is essential.Site selection Site selection is the most critical factor affecting pearloyster productivity and spat collection, as the oystersspend most of their growing time exposed to waterelements. Site selection must take into account important waterquality parameters like temperature, salinity and turbidity.
Optimum growth for P. margaritifera occurred at 23-28 0C Under 14 0C, larvae did not develop into veliger. Temperature also affects byssal attachment, with thehighest rate at 180C. Higher temperatures accelerate nacre deposition Salinity plays an important in larvae development. Survival of P. imbricata larvae was highest at 32 and 35 pptand at 29-35ppt, Turbidity also has an influence on growth and survival ofoysters.
SettlementSettlement of larval spat isaffected by various factors.Types of substrate also influence larvae settlement.Depth and chemical cues also affect spat recruitment,with an intermediate depth of 60-90 cmFeeding Optimum diet for P. margaritifera larvae was found to be1:1 mixture of Pavlova salina and Chaetoceros simplex anddried Tetraselmis suecica and commercial yeast L-10,Microfeast.
Stocking density For nursery culture at more than 100 individuals/ tray ,100 individuals/ tray after 3 months, 50/tray from the 4thmonth onwards At 30% stocking density is the optimum method forculturing P. margaritifera.Pearl culture techniquesAs the oyster grows, growth rate reduces progressivelyThe sooner the nucleus is implanted, the greater is therate of nacreous deposition, and the shorter is the timeto obtain a marketable pearl .
Problem and their solution in pearl culturePredationPredation of oyster spat andjuveniles can result in massive loss in productivity.Removal spat collectors at a shorter period 3-4 monthsinstead of 6 months, when mean size is 15mm and rearedin panel nets.Spat can be covered with mesh, and a mesh size of 3 mmimproved spat survival of P. maxima while not fouling soeasily.For juvenile culture, ear hanging and 24 pocket juvenilepanel nets were the preferred method, showing highestgrowth and lowest fouling and mortality from predators.
BiofoulingStunted growth and shelldeformities are caused by biofouling organisms likebarnacles and polychaete worms.Cleaning also affect survival rates, frequent cleaningevery 4 weeks resulted in high mortality forP.margaritifera.Diseases Oyster diseases have the potential to affect productivity. Pearl oyster P.maxima suffered high mortality inAustralia in 1996 due to the bloom of algaeTrichodesmium erythraeum.
However, another commonly occurring oyster disease,the reddening of abductor muscle caused by infection inP. fucata can be prevented by lowering the temperatureof water to less than 190C.GeneticsOne of the worries ofselection through inbreeding is loss in genetic diversity,which may lead to inferior growth and survival. The future of pearl oyster aquaculture lies in geneticselection, which can create faster growing oysters,resistance to diseases and production of higher qualitypearls.
ConclusionsPearl farming is an attractive business venture because ofthe high value of the final product. In recent years, pearl prices have fallen significantly,especially for smaller, lower-quality pearls. With the exception of the grafting process, pearl farmingis a relatively simple form of aquaculture because pearloysters do not require artificial feeds, complicated farmstructures or constant attention. If properly managed, pearl farming will not harm theenvironment and can increase the wild pearl oysterpopulation over a period of years. For these reasons, pearl culture may be the bestopportunity for business development.